Data communication systems are commonly used to transmit and/or receive data between remote transmitting and receiving locations. A central facet of any data communications system is the reliability and integrity of the data communicated. Ideally, the data, received at the receiving location is identical to the data transmitted from the transmitting location. Practically however, the effects of transmission echo (i.e., reflection of the transmitted signal such that it appears as part of the received signal) can cause the data which is being received to be corrupted or lost.
Signals received by a modem typically have a large dynamic range and may be affected by transmission echo. As modems transmit higher bandwidth signals over traditional twisted pair copper telephone lines, echo has a greater effect on modem performance. For example, a broadband Digital Subscriber Line (xDSL) operating over twisted pair copper wires, such as Asymmetric Digital Subscriber Line (ADSL) technology, uses signals transmitted at up to and over two megahertz which are typically attenuated by as much as 90 dB before being received by the modem. Additionally, the upstream and downstream data channels may use overlapping frequencies. When overlapping frequencies are used, the modem transmission echo must be compensated for, as it will have a greater amplitude than the received signal in the same frequency spectrum. In ADSL systems, the frequency spectrum is assigned to the various communications channels conveyed over the twisted pair using a splitter. The lowest 4 kHz of bandwidth is reserved for use by analog devices such as common telephones of the baseband Plain Old Telephone Service (POTS). A medium frequency range full duplex channel in the frequency range of 30 kHz to 140 kHz may be defined to carry upstream ADSL data and downstream ADSL data having a typical data rate in the 16 kbps to 640 kbps range. A high frequency channel that uses frequencies in the 140 kHz to 1104 kHz range carries downstream data having a typical data rate in the 1.536 Mbps to 6.144 Mbps range. Accordingly, the medium frequency upstream and downstream data channels used by ADSL modems may use overlapping frequency ranges.
Modems and other communication systems typically include a hybrid coupling circuit for connecting to the local loop which then connects to the telephone company central office. Additionally, the transmitted and received signals may be transmitted on overlapping frequencies as in the case of ADSL. Accordingly, an ADSL modem must incorporate a suitable hybrid circuit to attenuate or compensate for its own transmission echo in order to maximize the dynamic range of the receive path of the ADSL modem. The received signal is detected at the secondary side (the side of the transformer connected to the modem and hybrid circuit) of the line transformer. The transmitted echo signal (which has had its characteristics affected by the line transformer and the line itself) is inseparable from the received signal at the secondary side of the line transformer. In ADSL applications, it is likely that the local loop will be sufficiently long such that the effect of the telephone company central office devices or other devices connected to the ADSL modem will be negligible. However, the transmitted echo signal may oftentimes be affected by the telephone company central office devices, requiring compensation by the line compensation circuit of the balanced hybrid circuit.
Simultaneously with the detection of the received signal, a sample of the transmitted signal is taken from the output of the modem line driver and this sample is processed in order to obtain a replica of the transmitted echo signal. The replica of the transmitted echo signal is then subtracted from the received signal. Accordingly, the effectiveness of the attenuation of the transmission echo signal is related to how well the reconstructed echo signal matches the actual transmission echo signal. Accordingly, the received signal has an improved received to transmit echo signal ratio when the echo compensation circuit of the hybrid circuit closely replicates the transmission echo.
Prior art hybrids and circuits used to compensate for transmission echo signals in modems have several disadvantages, such as the use of a large number of components. Additional disadvantages of prior art devices include the use of active circuitry for transmission echo reconstruction and subtraction that can increase distortion and nonlinearity.